As well known to those skilled in the art, the sugar content (sweetness) of a given fruit is typically determined by experienced workers relying upon their senses of sight, touch, and smell. This process, which is based on the subjectivity of individual workers, is inherently inconsistent and inaccurate, thus reducing the reliability of fruit quality claims and rendering impossible the reliable selection of fruit according to a grade of sweetness. It takes several years for a worker to attain an acceptable level of competence to gauge the sweetness of a given type of fruit.
In an attempt to solve these problems, near-infrared spectroscopy has been used, with which internal qualities of fruit, such as sweetness or acidity, can be precisely determined. An operator positions in a light path a fruit sample selected from target fruits, and the intensity of light emitted from a near-infrared light source is sensed with respect to the positioned fruit, to determine a deflection value and a transmission value and thereby grade individual pieces of fruit according to sugar content. Here, the deflection value is the ratio of the intensity of light from the light source relative to the intensity of light deflected by the fruit sample, and the transmission value is the ratio of the intensity of light from the light source relative to the intensity of deflected light, and the transmission is a ratio of the intensity of light from a light source relative to the intensity of transmitted light. Preferably, the light source maintains a constant intensity of light, irrespective of the passage of time.
However, output of the light source used for measuring the sweetness of fruits is reduced as time passes while it is in operation, and accordingly, the error in measuring the sweetness increases as time passes. To reduce the error in measurement, light is radiated from a light source to a reference sample (compressed solid block of barium sulfide or Teflon) for measurement of light transmission or deflection at time intervals of one hour, and the energy of light transmitted through or deflected by the sample is set as the intensity of the light source. However, this method needs an additional step of measuring the light energy transmitted through or deflected by such a reference sample at regular time intervals, thus causing an inconvenience in operation.
In addition, the internal qualities such as sweetness or acidity of a fruit vary even within the same fruit according to portions thereof, that is, top and bottom, left and right portions, etc., and thus, it is difficult to precisely judge the internal qualities of fruits.